The present invention relates to a process for the treatment of a sealable film surface by means of a flame. The films are distinguished by good sealability, good printability and good antistatic behavior.
The flame treatment and the corona treatment are used for increasing the surface tension of polypropylene films. This makes the films printable, and wettable with water-based coating systems. While the corona treatment is used in the case of sealable and nonsealable films, the flame treatment is used mainly in the case of nonsealable films.
In the flame treatment of nonsealable films, the film is passed over a cooling roll below which a gas burner is arranged. The distance between burner and film surface/cooling roll is from 3 to 10 mm and is chosen so that the oxidation reactions on the polymer surface are maximum. The oxidation of the polymer surface results in the formation of oxidized polar groups, with the result that the surface tension of the film is increased in the desired manner. Preconditions for satisfactory treatment of the film are exact feeding of the film web on the cooling roll and a constant cooling roll temperature. The feeding of the film on the cooling roll is effected as a rule by means of a gummed nip roll.
It is taught that the treatment of nonsealable films should be carried out at a cooling roll temperature greater than 36.degree. C. (cf. "The base flame treatment process", H. Angeli/Fa. Esse Ci, 3rd International Meeting on the Plastic Surface Treatment, 1989, Narni, Italy). Below this temperature, the film surface exhibits water vapor condensation, which makes the film useless. A typical cooling roll temperature in the case of non-sealable film is 40.degree. C.
The flame treatment of nonsealable films has advantages over the corona treatment. These are a high surface tension which is constant as a function of time, little odor of the film and no effect on the back. A disadvantage is the very high thermal load of the film surface.
Sealable polypropylene films, which are sensitive to thermal load, are damaged in the flame treatment in such a way that they lose their sealability. The application of the process described above to sealable films is therefore not feasible. The thermal load of the sealable surface is too high, and the water vapor condensation described occurs when the cooling roll temperature is reduced.
In the case of sealable films, the use of the flame treatment with polarization is therefore taught (cf. "The polarized flame process", H. Lori/Fa. Esse Ci, 3rd International Meeting on the Plastic Surface Treatment, 1989, Narni, Italy). In this method, the burner is arranged above the cooling roll. A dc voltage is applied between the burner and the cooling roll, with the result that the ionized atoms in the flame are more highly accelerated and strike the polymer surface with higher kinetic energy. The chemical bonds within the polymer molecules are more readily broken, and the formation of free radicals takes place more rapidly. The thermal load of the polymer surface is lower than in the flame treatment without polarization. At the same flame temperature, a higher surface tension is achieved in the process with polarization than in the process without polarization. In other words, in order to achieve the same surface tension, a lower flame temperature is required in the process with polarization than in the process without polarization, with the result that the seal layer is protected. Owing to the changed thermal conditions, the cooling roll temperature can therefore be reduced to a temperature of 20.degree. C. in the flame treatment with polarization, without condensation occurring on the film surface.
The disadvantage of the flame treatment with polarization is the high static charge build-up on the film. The following adverse aspects are associated with this:
1. The film is more difficult to handle. The wound film is charged to such an extent that it can no longer be handled by the machine operator. There is a high risk of accidents, which cannot be eliminated by technical measures such as active or passive discharge of the film. PA1 2. The quality of the film is reduced. The high static charge build-up on the film results in optical defects, such as flash figures, shrink marks and indentations during its production. PA1 3. The processibility of the film is poorer. The high static charge build-up on the film leads to problems with unwinding and to material back-up during processing of said film on packaging machines.
It was therefore the object of the present invention to provide a process for the surface treatment of a film, in which a polypropylene film having good sealing properties and good antistatic behavior can be produced.